A MRAM is a nonvolatile memory that is considered a promising candidate for a universal memory from the viewpoint of high integration and high operating speed, for example. In a memory cell of a MRAM, a magnetoresistance effect element, such as a GMR (giant magnetoresistance) element or a TMR (tunnel magnetoresistance) element, is used as a storage element. These elements have a three-layer structure as a basic structure such that a non-magnetic layer is sandwiched between two ferromagnetic layers, i.e., a first ferromagnetic layer and a second ferromagnetic layer. One of the two ferromagnetic layers is a pinned layer with a fixed direction of magnetization, while the other is a recording layer with a switchable direction of magnetization. In the following, an example is described in which the first ferromagnetic layer is the pinned layer and the second ferromagnetic layer is the recording layer. The element has a low resistance when the magnetization direction of the pinned layer and the magnetization direction of the recording layer are parallel to each other (P state), or a high resistance when the magnetization directions are antiparallel (AP state). The ratio of such changes in resistance exceeds 600% at room temperature in the case of a TMR element in which MgO is used for the non-magnetic layer, as described in Non-patent Document 1, for example. The resistance ratio is known to be particularly high in the case involving coherent tunneling conduction via the Δ1 band, which is realized in a combination of a ferromagnetic material that contains at least one 3d transition metal element, such as Co or Fe, and MgO. In the MRAM, the resistance change is associated with bit information of “0” and “1”. As a method for writing bit information, a magnetization switching system based on spin injection has been proposed, as described in Non-patent Document 2. This system utilizes the phenomenon in which magnetization direction is changed by spin-transfer torque produced by a current caused to flow through the magnetoresistance effect element. When the current is caused to flow from the pinned layer to the recording layer, the magnetizations of the pinned layer and the recording layer become antiparallel, and the bit information is “1”. On the other hand, when current is caused to flow from the recording layer to the pinned layer, the magnetizations of the pinned layer and the recording layer are parallel, and the bit information is “0”.
However, in this system, a large current needs to flow through the magnetoresistance effect element itself at the time of writing. Thus, in the case of the TMR element with an insulator for the non-magnetic layer, the withstand voltage of an insulating layer becomes an issue. Further, as the reading speed is increased, higher magnetoresistance ratio values are required; generally, a high magnetoresistance ratio of 70% to 100% or higher is required. In the case of a GMR element in which an insulating layer is not used in the non-magnetic layer, there is the problem of long read time because of the small resistance ratio.
Patent Document 1, for example, discloses a MRAM of the magnetic domain wall motion type in which magnetic domain wall motion by a spin transfer effect is utilized. A magnetic domain wall is a region with a finite volume at the boundary of a plurality of regions called “magnetic domains” in which magnetization directions are aligned in a ferromagnet. Particularly, when the magnetization directions of two magnetic domains adjacent to each other are antiparallel, the magnetic domain wall at their boundary is referred to as a 180° magnetic domain wall. The magnetoresistance effect element of a memory cell of the magnetic domain wall motion type MRAM described in Patent Document 1 is provided with a pinned layer with fixed magnetization; a non-magnetic layer stacked on the pinned layer; and a magnetic recording layer stacked on the non-magnetic layer.
FIG. 1 shows a basic structure of a magnetoresistance effect element 100 of a memory cell of the magnetic domain wall motion type MRAM described in Patent Document 1, for example. FIG. 1(a) is a plan view, and FIG. 1(b) is a cross-sectional view. The magnetoresistance effect element 100 is provided with a pinned layer 101 which is a ferromagnet with fixed magnetization; a non-magnetic layer 102 stacked on the pinned layer; and a ferromagnetic magnetic recording layer 103 stacked on the non-magnetic layer. The magnetic recording layer 103 has a thin wire shape. Specifically, the magnetic recording layer 103 includes a magnetization switching region 104 with a region in which a magnetic domain wall with a finite width can move, the region disposed at a portion overlapping with the pinned layer 101 and the non-magnetic layer 102; and a pair of pinned magnetization regions 105 and 106 formed adjacent to the magnetization switching region 104. The pinned magnetization regions 105 and 106 are provided with pinned magnetization of opposite directions.
To the pinned magnetization regions 105 and 106, current supply terminals 107 and 108, respectively, are joined. To the pinned layer 101, a current supply terminal 107 is joined. At the time of writing, a write current is passed, via the current supply terminals 107 and 108, through the magnetization switching region 104 and the pinned magnetization regions 105 and 106 of the magnetic recording layer 103. In the magnetization switching region 104, a magnetic domain wall 110 is introduced. The magnetization switching region 104 have magnetization directions antiparallel to each other, with the magnetic domain wall 110 providing a boundary. When the write current flows, the magnetic domain wall 110 is moved such that the magnetization direction is changed in a region of the magnetization switching region 104 immediately above the pinned layer 101 and the non-magnetic layer 102. In the example of FIG. 1, when the current is passed from the current supply terminal 107 to the current supply terminal 108, the magnetic domain wall 110 is moved toward the pinned magnetization layer 105 such that the magnetization direction of the region of the magnetization switching region 104 immediately above the pinned layer 101 and the non-magnetic layer 102 becomes parallel to the magnetization of the pinned layer. When the current is passed from the current supply terminal 108 to the current supply terminal 107, the magnetic domain wall 110 is moved toward the pinned magnetization layer 106 such that the magnetization direction of the region of the magnetization switching region 104 immediately above the pinned layer 101 and the non-magnetic layer 102 becomes antiparallel to the magnetization of the pinned layer.
This system is advantageous in that, because no current flows through the non-magnetic layer 102 at the time of writing, the withstand voltage of an insulator does not need to be considered even when an insulator represented by MgO is used for the non-magnetic layer, so that a highly reliable structure can be obtained. At the time of reading, a read current smaller than the write current such that the magnetic domain wall 110 is not moved is passed through the pinned layer 101, the non-magnetic layer 102, and the magnetic recording layer 103 via the current supply terminal 107 and the current supply terminal 109, or the current supply terminal 108 and the current supply terminal 109. As a result, a current path structure similar to that of a GMR or a TMR is established, and the resistance change can be read as bit information.